1. Field of the Invention
This invention relates to a liquid crystal medium useful as a material for optical devices, particularly a liquid crystal medium having a wide temperature range of liquid crystal phase, a large dielectric anisotropy, and a large optical anisotropy. The invention also relates to an optical device that utilizes the liquid crystal medium, particularly an optical device that can be used in a wide temperature range and driven at a low voltage and is capable of obtaining a rapid electrooptical response.
2. Description of Related Art
Liquid crystal display (LCD) devices using liquid crystal compositions are widely used for display of clocks, calculators, word processors and so on. These LCD devices utilize the optical anisotropy and the dielectric anisotropy of liquid crystal compounds. The operation modes of LCD devices mainly include phase change (PC), twisted nematic (TN), super twisted nematic (STN), bistable twisted nematic (BTN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), vertical alignment (VA) and so on, which use one or more polarizers for display purposes. Also, many studies have recently been done to the mode where an electric field is applied to an optically isotropic liquid crystal phase to induce electric birefringence (Patent Documents 1-16, Non-patent Documents 1-3).
Moreover, wavelength tuneable filters, wavefront control devices, liquid crystal lenses, aberrational correction devices, aperture control devices, optical head devices and so on that utilize the electric birefringence of a blue phase as one of the optically isotropic liquid crystal phases have been proposed (Patent Documents 10-12).
According to the driving mode, LCD devices can be classified into passive matrix (PM) and active matrix (AM) types. The PM type is further classified into static type, multiplex type and so on, and the AM type is classified into thin film transistor (TFT) type and metal insulator metal (MIM) type, etc.
Each of these LCD devices contains a liquid crystal composition with suitable properties. To improve the characteristics of an LCD device, it is preferred that the liquid crystal composition has suitable properties. General properties necessary for a liquid crystal compound as a component of a liquid crystal composition include:    1) chemical and physical stability,    2) a high clearing point (liquid crystal phase-isotropic phase transition temperature),    3) a low lower-limit temperature of liquid crystal phase (nematic phase, cholesteric phase, smectic phase, and optically isotropic liquid crystal phases like blue phase, etc.),    4) good compatibility with other liquid crystal compounds,    5) a suitable dielectric anisotropy, and    6) a suitable optical anisotropy.Particularly, in view of lowering the driving voltage, a liquid crystal compound being large in the dielectric anisotropy and the optical anisotropy is preferred for an optically isotropic liquid crystal phase.
When a liquid crystal composition including a liquid crystal compound with chemical and physical stability (the 1st property) is used in an LCD device, the voltage holding ratio can be improved.
In addition, a liquid crystal composition including a liquid crystal compound having a high clearing point or a low lower-limit temperature of liquid crystal phase (the 2nd and 3rd properties) can have a wide temperature range of nematic phase or optically isotropic liquid crystal phase, and therefore can be used in display devices in a wide temperature range. To exhibit better properties that are difficult to exhibit by a single compound, a liquid crystal compound is usually mixed with a number of other liquid crystal compounds to prepare a liquid crystal composition for use. Therefore, a liquid crystal compound used in an LCD device preferably has good compatibility with other liquid crystal compounds (the 4th property). Recently, LCD devices with superior properties, especially display performances like contrast, display capacity, response time and so on, are required. In addition, regarding the liquid crystal material used, a liquid crystal composition with a low driving voltage is required. Also, in order to drive at a low voltage an optical device that is driven in an optically isotropic liquid crystal phase, it is preferred to use a liquid crystal compound with large dielectric anisotropy and optical anisotropy.
On the other hand, an optically isotropic liquid crystal composition containing a compound of formula (1) of this invention in an amount of 32 wt % to 85 wt % is good in the driving voltage, response time, contrast, and low-temperature preservation stability, or has a good balance between these physical properties. A composition containing a compound of formula (1) of this invention has been disclosed in Patent Document 15. However, in those Patent Documents, the content of the compound of formula (1) is low, so it is difficult to simultaneously attain low-voltage driving, and good low-temperature preservation stability or high contrast or high-speed response.